Gray Lord had the Ash demon masks, to me that should give us a hint that he could in fact be a demon.Imo he's an evolved Ash demon, or a conglomeration of many Ash demons . So he could be of a higher class than the Ash demons, but powerful enough to become a commandment.
Ash demon Hendrickson is very different from the real Ash demon, he has horns while the Ash demon doesn't. In fact his appearance was closer to that of Cernnunos, the Deer God. Fraudrin's real form seems to be ethereal, with rough spiky edges like Mel's shadow that was shown in the beginning of the manga.
Mel's shadow:
Fraudrin's real form:
So what species is he?To me both are very similar, so imo he is a demon alright, same as Mel. I think you're all getting hyped by the idea that other species would join the demon clan, when it could be just those two we saw.
The first part I mentioned is that she like Fraudrin, like Gray Lord seems to have "demonic" attributes. She also transforms her body, similar to Gray Lord. So why can it not be that she could be a full demon? What other species has those attributes, because if we look at Gloxinia and Dolor they seem to have retained the attributes of their clan. The tentacles look like one of Basquias's forms, Gloxinia's ST, as for Dolor other than the double arm and the green blue skin(which could be the result of Heavy metal) he looks like any regular giant we've met so far.
You're discussing the possibilities that Mera and the others would come from other clans, but none of their attributes really fit those of said other clans. Which clan can use his tongue like Merascylla did? I don't think we've seen any do that.
So imo Glodolor could be a red herring, and other members of the commandments might not be members of other clans transformed into demons. As for Mel he never answered the question clearly, but to me I'm fairly sure he doesn't equate being a demon with being evil. Which is why it is in his eyes unnecessary to answer King's questions, and more importantly I have a feeling he's hiding the truth because he's related to the demon king, not because he's a hybrid. He is a demon, the vampires recognized him as much, but he would never answer King's questions because like I said for him it doesn't matter that he's a demon, and because of some secret in his past. Being an hybrid is not the only option here.
Wolf eyes lol. Plenty of characters in NnT have them, without having to be related to wolves. It's just how Suzuki draws some eyes, since he has a lot of types, and if we're going to take eyes as a clue, then Helbram must have also been a werewolf

Derrierie's eyes:
Helbram's:
They're pretty close.And what's more funny is that the real were(fox) while he's in human form doesn't have them.
Going by your logic Derrierie could be a werewolf because she has wolf shaped eyes, so in turn Zhivago who is a werefox should also have fox shaped eyes,and yet he doesn't!

In fact his eyes in human and werefox form are completely different.
So I don't see their attributes being representative of any other clan in particular, especially Monspiet, from which we've seen nothing that could hint he's a vampire. These could be found on random character, and if I may add it's too slim to make a real and sound theory.